PROJECT SUMMARY/ABSTRACT Delay activity is neural activity that occurs within a few seconds between the encoding of relevant stimuli and memory retrieval. Delay activity is a key component of working memory, a cognitive process that allows us to hold information online and manipulate it to realize immediate and future goals. While delay activity has been extensively studied in animals and humans using a variety of neuroscientific recording methods and different levels of data analysis, there remain several gaps in understanding the mechanisms by which delay activity contributes to both short- and long-term memory. Recent studies question the long-standing theoretical framework that elevated and sustained delay activity is the basis for memory maintenance. Findings from these studies suggest that delay activity is often transient or burst-like, and that unattended stimuli may even be maintained in activity-silent hidden states. These new findings motivate the experiments outlined here to understand how observable delay activity contributes to memory formation. This proposal will combine high temporal resolution electroencephalographic and high spatial resolution functional MRI methods to test in human participants the overarching hypothesis that early and late components of delay activity contribute to memory formation. The first aim will study the effects of varying the delay interval duration and content on delay period activity and memory using the techniques of electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). The hypothesis is there will be a temporal correspondence between EEG delay period synchronization and BOLD-fMRI amplitude, with a positive relationship of both to subsequent memory. The second aim will study the effects of delay period EEG synchronization/desynchronization and hippocampal-cortical fMRI connectivity on memory. The hypothesis is that early synchronization and later desynchronization in delay periods reflects hippocampal-cortical binding and cortico-cortical reactivation preceding memory retrieval. The third aim will study the effects of EEG delay activity and subsequent sleep spindles on memory. The hypothesis is that more delay activity will elicit long-term memory consolidation processes during a post-learning sleep period. Together the completion of these three aims will contribute to a basic understanding of the role of delay activity in memory. The outcomes will provide valuable insight into why certain mental health and neurological disorders, which include aberrant delay activity, lead to complex patterns of both short- and long-term memory impairments.